1. Field of the Invention
This invention relates to an optical pickup device adapted to record and/or reproduce data signals on or from an optical disk.
2. Related Prior Art
There is so far utilized an optical pickup device which is provided with a semiconductor laser element as a light source and in which a light beam emitted by the semiconductor laser unit is converged on a signal recording surface of an optical disk by a suitable optical system to record and/or read data signals on or from the signal recording surface.
This type of optical pickup device is provided with a photosensor, such as a photodiode, for detecting the light beam reflected from the signal recording surface. Based on the photosensor output, read-out signals of the data signals, focusing error signals and tracking error signals are formed. Based on these error signals, focusing servo and tracking servo control operations are performed, such that, even if the optical disk is subjected to de-centering or deviation from true planarity during its rotation, the light beam projected on the signal recording surface may be converged correctly on the recording track formed on the signal recording surface.
In such type of optical pickup device, it has been proposed to reduce the size or the focal distance of the optical device adapted for converging the light beam on the signal recording surface, such as an object lens, to shorten the optical distance between the semiconductor laser element and the signal recording surface and to thereby reduce the size of the pickup device.
However, in the optical pickup device in which the distance between the semiconductor laser element and the signal recording surface is shortened, so-called return light to the semiconductor laser element increases. The return light means the light beam fraction which is reflected by the signal recording surface of the disk and returned to the semiconductor laser unit. In general, an increase in the return light results in increased noise due to increased light intensities, which in turn gives rise to various inconveniences, such as fluctuations in the light emitting power. Hence, it may become occasionally impossible to perform correct write and/or read operations for data signals.
A so-called gain waveguide type laser element is suited to be used as the semiconductor laser element of the optical pickup unit which is reduced in size. The semiconductor laser element may be classified into a so-called refractivity index waveguide type element oscillating in a single mode and a so-called gain waveguide type element oscillating in a multiple mode. By comparison, the gain waveguide type laser element is subject to the noise due to return light to a lesser extent than the refractivity index waveguide type element and hence may be used advantageously as the above mentioned laser element.
However, the gain waveguide type laser element exhibits a larger astigmatic difference than the refractive index waveguide type laser element. The light beam emitted by the semiconductor laser element is subject to astigmatism due to the astigmatic difference proper to the semiconductor laser element. For this reason, the beam spot formed on the signal recording surface is not truly circular as shown at (b) in FIG. 1 but assumes the shape of ellipses having their long axies extending in mutually perpendicular directions, depending on the de-focusing directions, as shown at (a) and (c) in FIG. 1.
If the contour of the beam spot 51 is changed as shown at (a) and (c) in FIG. 1, as a result of de-focusing, the surface measure of the beam spot 51 on the recording track t.sub.R being thus changed, the minimum tracking error signal level position and the maximum RF signal level position are not coincident with each other nor with the focusing position at which jitter of read or written data signals becomes least, as shown in FIG. 2, so that non-symmetricities of the tracking error signal level and the RF signal level are caused with respect to the focusing position.
As a result of these non-symmetricities of the tracking signal level and the RF signal level with respect to the focusing position, it may become occasionally impossible to effect focusing or tracking servo operations on occurrence of de-focusing due to de-centering or deviation from planarity of the optical disk.
In the optical pickup device employing the gain waveguide type laser element, such astigmatism is corrected by a plane parallel glass which is inclined a predetermined angle with respect to the optical axis of the light beam and which is placed on a light path along which the light beam is dispersed or converged. This aberration correction plate is provided, for example, between the semiconductor laser element and a beam splitter which is adapted for transmitting the light beam reflected by the signal recording surface towards the photosensors.
In the above described optical pickup device, it has been proposed to provide a light emitting/receiving composite unit to reduce the size of the device further. This composite unit is formed by the semiconductor laser element and split detectors on a unitary semiconductor substrate, these detectors functioning as the photosensors. On the semiconductor substrate, there is provided a beam splitter prism whereby the light beam emitted by the semiconductor laser element is caused to exit the semiconductor substrate at a predetermined angle with respect to the semiconductor substrate so as to be projected on the signal recording surface of the optical disk, the light beam reflected by the signal recording surface being received by the split detectors.
For reducing the size of the pickup device, these detectors employed in the light emitting/receiving composite unit are composed of first to third juxtaposed light receiving elements each having a rectangular light receiving surface. The state of the light beam received by these detectors, for example, the state of energy distribution on a cross-sectional surface of the flux of the light beam, is changed as a function of the relative position between a beam spot formed by the convergence of the light beam from the semiconductor laser element on the signal recording surface and the recording track on the signal recording surface. The tracking error signals are formed by comparing the outputs of the first and third light receiving elements provided on both sides of the photodetectors. On the other hand, the focusing error signals are formed by comparing the output of the second light receiving elements at the central portions of the photodetectors with the outputs of the second and third light receiving elements.
However, it is not possible with the optical pickup device employing the light emitting/receiving composite unit formed by the detectors to compensate for the above described non-symmetricities of the tracking error signal with respect to the focusing position, even with the use of the aberration correcting plate formed by a plane parallel glass inclined at a predetermined angle with respect to the optical axis of the light beam.
Thus, when forming the tracking error signal in the above described split detectors, the second light receiving element at the center of the detectors proves to be an insensitive zone. Consequently, the portion of the light beam received by the detectors which is in the vicinity of the optical axis of the light beam cannot be received. In regard to aberration correction by the aberration correction plate, although astigmatism can be compensated by the plate, coma aberration is likely to occur as a result of compensation for astigmatism. The effect of coma aberration becomes more pronounced in a region further away from the optical axis of the light beam. Hence, the effect of coma aberration on the output signal becomes larger in the photodetector having an insensitive central zone, such as the above described detectors, thus giving rise more readily to the above described non-symmetricities of the tracking error signal with respect to the focusing position.
It is also difficult with the optical pickup device employing the aberration correction plate for correcting the astigmatism to reduce the size of the pickup device on account of the spatial requirement for the aberration compensation plate.